Sound systems can be broken down into three general components: an input device, such as a microphone; a processing system; and an output device, such as a speaker. Sounds are picked up by the microphone, transmitted to the processing system where they are processed, and then projected by the speaker so the sounds can be heard at an appropriate distance. Both the microphone and the speaker are generally considered to be transducers.
A transducer is a device that transforms one form of energy to another form of energy. In the case of a microphone, sound energy, which can be detected by the human ear in the range of 20 Hertz to 20,000 Hertz, is transformed into electrical energy in the form of an electrical signal. The electrical signal can then be processed by a processing system. After the signal is processed including amplification, the speaker transforms the electrical energy in the electrical signal to sound energy again.
This sound energy from the speaker (or a portion of this sound energy) may in turn be picked up by the microphone, and returned to the sound system. This is known as feedback, and in particular acoustic feedback. The presence of acoustic feedback may preclude the useful operation of hearing aids and other such sound systems (i.e., those with sound-sensing and sound-producing transducers). Even if the level of the feedback is sufficiently low, it may distort the production of sound at the speaker. At another level, the feedback may cause ringing effects that tend to reduce the intelligibility of speech. At high levels of feedback, a high-pitched squealing tone can be heard that dominates and excludes all other desired sounds produced by the sound system.
These effects are frustrating to users of sound systems in general, but are particularly debilitating for users of hearing aids since these users depend upon such aids to maintain their ability to communicate.
Several methods have been tried to eliminate unstable feedback. These include: 1) reducing the system's gain at and around the frequency of the feedback; 2) varying the phase of the system; and 3) using a filter to eliminate the feedback signal. The first method is undesirable; since feedback may occur at several or variable frequencies, the method requires a burdensome number of filters to isolate frequency regions of the feedback; in certain instances, the method yields audible artifacts in the output. The second method is also undesirable; phase shifting to eliminate feedback at one frequency is likely to produce feedback at a different, previously stable, frequency; this method also may produce audible processing artifacts. The third method represents a more desirable approach. However, many of the current implementations of the third method add other problems of their own.
In the third method, because of the variations in the feedback path over time, the filter itself should be sensitive to feedback variations. Filters used in hearing aids, for example, must be sensitive to mouth movements, use of a telephone, etc. Sensitivity of the filter can be adjusted by using three current different implementations: 1) by interrupting and injecting a signal into the feedback path as in U.S. Pat. No. 4,783,818; 2) by injecting a noise signal to accommodate changes in the acoustic coupling as in U.S. Pat. No. 5,259,033; and 3) by relying on ambient signals as in U.S. Pat. No. 5,402,496. The first implementation adds audible and annoying sounds to the listener. The second implementation requires a long duration for providing the filter with needed information, and thus exposing the listener to a longer duration of unstable feedback. The third implementation can be corrupted by persistent correlations in the ambient signals. These correlations limit the ability of the filter to cleanly and effectively inhibit feedback.
Thus, what is needed are systems, devices, and methods to inhibit undesired feedback in sound systems.